As is known in the art of electrophotographic imaging, a photoconductive surface in an electrophotographic imaging system is first charged to a uniform potential and then is "exposed" to an image to be reproduced by the scanning of a laser beam thereacross. The photoconductor thereby obtains an electrostatic latent image that constitutes a matrix of discharged pixels on the photoconductor's surface. In a black and white printer, the photoconductive surface is generally developed using a black toner that adheres to the discharged pixel areas to form the image. Thereafter, the toned photoconductive surface is then carried to a transfer station where the image is transferred to a media sheet.
In a multi-color printer, successive images are developed employing different color toners supplied from corresponding toner modules. Color printing is normally done with yellow, cyan and magenta toners that are applied, in registration, during successive rotations of the photoconductive surface. The printer also generally includes a toner module with black toner. The developed color image is then transferred from the photoconductive surface to a media sheet. As is understood in the art, an alternative method to that described above is to use an intermediate medium wherein the individual color planes are transferred from the photoconductive surface to the intermediate medium. Once all the color planes have been transferred to the intermediate media, the composite image is transferred to the final media sheet. Heat is usually applied to permanently fuse the image to the media sheet in order to form a completed multi-color print.
The electrophotographic process is based on the electrostatic attraction of charged toner particles for opposite (or relatively opposite) sign charge on a photoconductor material on which an image has been formed. The surface of the photoconductor may be positive relative to the negative charge on the toner particles, or vice versa.
In most electrophotography, the desired image is developed on the photoconductor (often an organic photoconductor, OPC) using the customary principles of discharge area development (DAD). For DAD, the OPC must be capable of charging to the same sign of electrical potential as the formal charge on the toner. For example, when the OPC charges positively, the toner must have a positive charge. The concept also works when the OPC and toner are both negatively charged. DAD is preferred because the printed dots are oval or elliptical, which gives better print quality in terms of edge smoothness of the printed images.
In DAD printing, the entire surface of the OPC is charged up to a certain potential, the laser discharges those areas to be imaged ("write black"), and toner particles, having the same sign charge as the still-charged area of the OPC, are brought into contact with the OPC. The toner particles are electrostatically repelled by the same-sign charged areas and attracted to the discharged image area. Thus, the toner is electrostatically deposited onto the OPC. If the toners are transparent enough to the laser light, this process can be repeated until as many color planes as desired are overlaid.
Alternatively, a toner may be used which has an opposite sign charge to the photoconductor material and results in charge area development (CAD). Using a CAD process, the laser must discharge the area that is NOT intended to receive the toner (write white). The toner, which is of opposite sign compared to charged imaged areas, is electrostatically repelled by the discharged area and attracted to the opposite-sign charged areas. This mode is less favored because the dots formed by the toner are the "inverse" of the laser image and consequently have points or cupped edges. Thus, image edges formed by these pointed spots will be rougher and print quality is negatively impacted.
There are many interrelated reasons for choosing a given photoconductor or a given colorant material for the electrophotographic (EP) process, and occasionally the combination of these considerations forces a compromise in the materials set which is not ideal. Considerations regarding the photoconductor, such as production cost, environmental regulations, dark decay characteristics, durability, and the like, impact on the material of the photoconductor, the method of printing that will be used and the sign of electrical charge that must be associated with the toner. In a single-color monochrome printer or copier, this is usually not a major issue.
With multi-colored systems, it is advantageous if all the colored toners are of the same sign, thereby allowing the same imaging technique to be used for all color planes in the process. However, colors are developed using pigments that can have very different molecules each with unique chemistries. These unique chemistries may have considerable impact on the interaction of the pigment with the stabilizing and fusing resins, dispersing media, charging agents and other additives used in the toner formulation. A pigment considered ideal for color and print quality may be rejected because its chemistry renders it incapable of interacting satisfactorily with other components of the toner formulation. A serious situation arises when the pigment cannot satisfactorily accept the charging agent, or when the pigment itself charges to the sign opposite of the other pigments chosen. Therefore, requiring all members of the toner set to have the same sign charge can stand in conflict with other considerations. Such conflicts can result in compromises in color, print quality, toner stability, or the like.
Therefore it is the objective of the present invention to allow toners in a multi-color set to have either sign charge on them, thereby permitting choice of photoconductor regardless of the sign of the electrical charge developed on the photoconductor.